Dialers are used to interface station apparatus with a line extending to the public switched telephone network (PSTN). In one application of dialers, they store access numbers of preferred carriers. Various carriers may be preferred to save cost, due to bandwidth provisions or noise specifications, due to corporate agreements, etc.
To provide the dialing function, the dialer detects an offhook condition of the station apparatus, splits the line, receives digits dialed from the station apparatus, analyzes at least some of the dialed digits, and then either adds digits to the dialed digits and redials them to the PSTN, or substitutes and redials a different set of digits.
Due to the above requirements, dialers have required an auxiliary (but typically built-in) power supply. In a corporate environment where there is a telephone room which houses a PABX, a mains source is usually close by. Personnel are usually available to program the dialer as to the alternate or auxiliary directory numbers to dial. However, in a domestic residential environment, mains power sources are not necessarily available close by the dialer. For example, it may be desirable to locate the dialer close to where a subscriber's line enters the residence, which may be beside the basement joists where there is no mains power socket.
In addition, where such a dialer is purchased from a retailer, the customer may not be sufficiently skilled to program the dialer.
In a residential environment, if the dialer would be line powered, there may not be sufficient line voltage to operate the dialer reliably, and the result would be attempted operation of the dialer in a condition which would be against regulations of the authorities. Typical residences do not have the equipment or skill set to determine whether the line voltage is sufficient to meet regulations.
A problem with telephone line powered equipment involves maintenance, i.e. how they are to be programmed and how the programs are to be updated or changed. If the unit is to be locally programmed, this requires the presence of a person who is capable of programming, which is costly to the service provider, who may have to travel to the customer premises to perform the programming.
Remote programming had been made by counting the number of calls made and then dialing a remote maintenance site after a predetermined number of calls have been made. Another method has been used by having the maintenance site dial up the site to maintain it after it answers the line.
In calling up the line powered dialer from a remote maintenance site, the audible ringing will be heard from the customer's station apparatus. If a call is made to a remote maintenance site following a certain number of calls, this can occur during a heavy traffic period on the line. Thus these methods can disturb or inconvenience the customer, which is particularly undesirable in a residence application.
Another design concern for dialers is to meet the regulatory requirements for loop voltage and current.
Another design concern for dialers is the prevention of noise outside the telephone audio band (e.g. above 3400 Hz) from reaching the telephone line, and the passage of audio signals between the subscriber side of the dialer and the telephone network side of the dialer during the interval when the dialer is outdialing digits.
Typically some type of filter has been used to prevent the out of band noise from reaching the telephone line. However, this approach tends to be limited in effectiveness because it is difficult to achieve attenuation greater than 30 to 40 dB.
A prior art approach to determine the need for an external power supply included the measurement of the voltage across the telephone line with the telephone offhook, and the use of the product in operation to determine whether it will function. This is an hit or miss situation, which can produce an unhappy customer since the customer may consider that he has purchased an inoperative product. In addition, measurement of the voltage requires the use of test equipment which may not be available in the average residence.
Sometimes a light emitting diode is used to indicate proper operation voltage. However a light emitting diode consumes electrical current which reduces the line length over which the dialer will operate.